This invention relates to an apparatus for treating a flue gas to remove nitrogen oxides and sulfur oxides from a combustion flue gas discharged from boilers, industrial furnaces, gas turbines, and combustion facilities for treating wastes. Nitrogen oxides will be hereinafter referred to merely as NOx, and sulfur oxides merely as SOx.
Recently in Japan, fuel species for combustion is changing from heavy oil to coal due to tight supply of heavy oil to reduce the petroleum dependency, and coal-fired boilers of large capacity for power plants are now under construction for utility companies. However, coal as fuel has poor combustibility as compared with petroleum fuel, and NOx, SOx and uncombusted matters are liable to be emitted into the flue gas from coal-fired boilers. To reduce emission of NOx, slow coal combustion has been carried out by dividing the combustion flame into sections, or recycling the flue gas, conducting the combustion at two stages or removing NOx within the furnace before emission to the outside.
In the coal-fired power plants, the boilers are operated not always under a full load, but variable load of 75%, 50% or 25% full foad, or the boiler operation is discontinued, for example, according to the so called Daily Start-Stop schedule (which will be hereinafter referred to merely as "DSS schedule") or the so called Weekly Start-Stop schedule (which will be hereinafter referred to merely as "WSS schedule"). That is, the coal-fired power plants operatable under such intermediate load have been in keen demand.
On the other hand, a combination of a gas turbine of good startup characteristics with a waste heat recovery boiler, i.e. the so called combined plant, is now going to be contructed to meet the power generation under the intermediate load besides the coal-fired boilers, and is to be operated only in the daytime from Monday through to Friday according to the DSS or WSS schedule to meet the large power demand, and the operation is stopped at night or on Saturday or Sunday or holidays.
However, according to more stringent restriction of the NOx or SOx concentration of flue gas, power plants operable under the intermediate load, which are provided not only with the conventional combustion improvement, but also with an apparatus for treating the flue gas such as an apparatus for removing NOx by catalytic reduction by dry process, i.e. an apparatus for removing NOx with NH.sub.3 as a reducing agent in the presence of a catalyst, or an apparatus for removing SOx have been now constructed in increasing numbers.
FIGS. 14 and 15 show schematic flowsheets of apparatuses for treating a flue gas from a boiler based on a typical equilibrium draft system, provided with an apparatus for removing SOx and an apparatus for removing NOx according to the prior art.
In FIG. 14, an apparatus for removing NOx is provided on the downstream side of an economizer, and thus this system is called "After-Economizer System".
In FIG. 15, an apparatus for removing NOx is provided on the downstream side of an apparatus for removing SOx, and thus this system is called "After-DeNOx system".
In FIG. 14, the air pressurized by a forced draft fan 1 is heated through heat exchange with the flue gas passing through a flue gas duct 3 at an air heater 2, and is used as a combustion air in a boiler 5 through an air duct 4. The combustion flue gas from the boiler 5 is led to an apparatus for removing NOx through an economizer 6, and on the way ammonia (NH.sub.3) as a NOx-reducing gas is injected into the flue gas through an injection pipe 8. The waste heat of flue gas treated in the apparatus for removing NOx through the flue gas duct 3 is subjected to heat recovery in the air heater 2 and then to dust removal in a dust collector 9. Then, the flue gas is led to an apparatus for removing SOx 13 through an induced draft fan 10, a gas-gas heater 11, and a fan 12 for the apparatus for removing SOx. In the apparatus for removing SOx, the flue gas must be treated at a lower temperature (usually the treatment is carried out by wet process or by semi-wet process). When the treated flue gas after the removal of SOx in the apparatus 13 is discharged as such from a stack 14, there is a problem of white plume. Thus, the gas-gas heater 11 is provided to reheat the treated gas with the untreated flue gas. The gas-gas heater 11 is to lower the temperature of the flue gas to the apparatus for removing SOx 13 by wet process to reduce the evaporation of water during the water cooling.
According to the said After-Economizer System, the apparatus for removing NOx 7 is provided in a most suitable temperature zone for removing NOx at the outlet of boiler 5 (economizer 6), and also the apparatus for removing SOx 13 is provided in the most suitable temperature zone for removing SOx.
On the other hand, according to the After-DeNOx system, the apparatus for removing SOx 13 is provided on the upstream side of the apparatus for removing NOx 7 to remove SOx at first, and then NOx, as shown in FIG. 15. That is, the flue gas from the fan 12 for the apparatus for removing SOx 13 is subjected to SOx removal in the apparatus 13, and led to a heater 17 through a gas-gas heater 11, a gas-gas heater 15 and a fan 16 for the apparatus for removing NOx, and in the heater 17 the treated gas after the SOx removal is heated to the most suitable temperature range of 300.degree. to 400.degree. C for removing NOx. Then, the treated flue gas is mixed with NH from the NH.sub.3 injection pipe 8, and subjected to NOx removal in the apparatus 7, and discharged to the atmosphere from the stack 14 through the gas-gas heater 15.
However, both of these conventional After-Economizer System and After-DeNOx system have the following problems. That is, in the apparatus for treating a flue gas according to the After-Economizer System, the NOx removal is carried out before the SOx removal, and thus NH.sub.3 leaked from the apparatus for removing NOx 7 tends to be combined with SO.sub.3 present in the flue gas, and the resulting ammonium bisulfate deposits on the elements of the air heater 2 to clog the clearances between the elements. Furthermore, the dusts recovered in the dust collector 9 tend to be contaminated with the leaked NH.sub.3 to bring about various problems. Also, the waste water from the apparatus for removing SOx also tends to be contaminated with the leaked NH.sub.3 to increase the N content of the waste water to cause the so-called enriched nourishment phenomenon.
In the After-Economizer System, the combustion air from the air duct 4 leaks into the flue gas in the air heater 2 (usually the leakage is estimated to be about 7%), and, thus, the capacity of the apparatus for treating a flue gas is increased, and consequently the capacity of the drafting system, including the forced draft fan 1, forced draft fan 10, fan 12 for the apparatus for removing SOx, etc., is also increased. This leads to an increased power consumption.
To solve these problems, various types of an apparatus for treating a flue gas have been contemplated. For example, the dust collector 9 is provided on the upstream side of the apparatus for removing NOx 7, where the problem of contamination of dusts with NH.sub.3 can be solved, but the problem of contamination of the waste water from the apparatus for removing SOx 13 with NH.sub.3 is not solved. Thus, the After-DeNOx system where the apparatus for removing NOx 7 is provided on the most downstream side in the apparatus for treating a flue gas to solve the problems of leaked NH.sub.3, as shown in FIG. 15, is employed.
According to the After-Economizer System, the dusts recovered in the dust collector 9 are not contaminated with the leaked NH.sub.3 and thus have a high utility in application to cement materials, etc. or have no smell of NH.sub.3 or contamination of rain water, etc. when discarded.
Furthermore, the waste water from the apparatus for removing SOx is not contaminated with the leaked NH.sub.3 and thus no denitrization treatment is required. The flue gas to the apparatus for removing NOx 7 contains no sulfur components and thus the life of an NOx-removing catalyst can be maintained for a longer period of time. However, the After-DeNOx system has a problem of heat utilization, i.e. large heat loss on the whole, because in the After-DeNox system shown in FIG. 15 the gas temperature at the outlet of the fan 12 for the apparatus for removing SOx 13, i.e. at the inlet of the apparatus 13, is 140.degree. to 150.degree. C., whereas the treated gas at the outlet of the apparatus 13 is cooled to 40.degree. to 50.degree. C., and thus the flue gas must be heated to a suitable temperature of 300.degree. to 400.degree. C. for the NOx removal through the heater 17 before the flue gas is led to the apparatus for removing NOx 7. Thus, it is a key of the After-DeNOx system how effectively the heat recovery should be carried out between the SOx removal and the NOx removal. To this end, heat recovery has been so far carried out with two gas-gas heaters 11 and 15, among which the gas-gas heater 15 has a larger influence upon the operating cost. The gas-gas heater 15 is to heat exchange the flue gas at the inlet of heater 17 with the treated gas at the outlet of the apparatus for removing NOx, and the heating duty of the heater 17, i.e. the amount of fuel used, depends on the heat exchange efficiency of the gas-gas heater 15. The higher the heat exchange efficiency, the higher the flue gas temperature at the inlet of heater 17 and the smaller the temperature difference from the treated gas at the inlet of the apparatus for removing NOx 7. That is, the fuel to the heater 17 can be saved, but the heat exchange efficiency naturally has a limit. Even in case of using a gas-gas heater 15 of high heat exchange efficiency, the temperature increase of flue gas to the heater 17 is only by 30.degree. to 50.degree. C. The heater 17 is provided on the downstream side of the apparatus for removing SOx 13, and thus only fuel of low sulfur content must be used. In other words, the fuel cost will be higher, and even the temperature increase by 30.degree. to 50.degree. C. means a large increase in operating cost when applied to a large volume of flue gas from boilers, etc. from commercial power plants. That is, how to recover the heat from the combustion of such high cost fuel to decrease the fuel cost in the entire plant has been a problem to be solved.